It is known in the art of engine cooling to use a fan connected to an engine powered driving member such as the engine water pump via a shaft or belt to increase the airflow over an associated engine heat exchange device. The discovery some thirty years ago that an engine only needs to be cooled about ten percent of its operating time led to the design and production of a fan clutch utilizing fluid shear to couple the fan to the driving member. This fan clutch operates by passing a viscous silicone fluid between several sets of grooves and lands. The grooves and lands are separated several thousandths of an inch in which the silicone fluid is allowed to flow. The specific spacing is determined by the viscosity of the fluid and the power to be transmitted. This fluid serves as a medium for transmitting torque between the driving member and the fan.
Conventionally, there are two basic types of fluid shear fan clutches. One type operates in either an on or off mode. The other operates in a modulating mode between totally on or totally off. The on/off fan clutch performs well but because of its mode of operation it does not provide optimum cooling. For this reason it is used less often than the modulating fan clutch. The on/off fan type clutch has further limitations that are also found in the modulating type clutch. Neither unit when fully engaged is able to transmit better than eighty five percent of the input R.P.M. into fan rotation. The ability to transmit close to one hundred percent of the input R.P.M. would allow for more efficient cooling.
Both types of fan clutches have common areas where improvements can be made, outside of the range of R.P.M. that can be reached. The first is in lowering the fan clutch operating temperature. Current fan clutches operate by shearing viscous silicone fluid. This shearing process generates very high temperatures, as high as 780 degrees F. in some extreme cases and 480 to 500 degrees F. on a regular basis. Operation under these high temperatures causes less efficient clutch operation. Due to these high temperatures, aluminum and steel are the only cost effective materials which can be used internally. The grooves and lands which are required to shear the silicone fluid must have very close tolerances and thus require precise manufacturing. Therefore, the grooves and lands are machined out of a diecast aluminum blank. This machining is very time consuming and expensive. Since the internal temperatures of the clutches are so high, a less expensive manufacturing method, such as injection molding of a thermoplastic is not possible.
The modulating fan clutch is designed to increase the ratio of input R.P.M. to output R.P.M. as the temperature in the engine compartment rises. The outer limits of modulating operation resemble the on/off fan clutch, but between these limits the modulating fan clutch does not always operate as designed. This operational limitation is due to the reservoir and valve which control the fluid in the fan clutch. The valve must let fluid out of the reservoir at a precise flow rate in order to place the correct amount of fluid between the grooves and lands and thus obtain the desired R.P.M. ratio.
A problem that both types of fan clutches have is when the engine is not running. When the engine is not running the valve on the reservoir leaks fluid into the grooves. Upon engine ignition the fan clutch is thus engaged even though the engine is considered cold. This is not desirable because automobile owners hear the fully engaged fan clutch over the idling engine and think that there is a problem.
Furthermore, because of fan engagement at start up and the resulting increase in engine warm up time, the higher level of vehicle emissions occurring at start up are maintained for a longer period of time.